OCR A Level Biology Photosynthesis

What are the 3 photopigments found in the group of chlorophyll?

1. Chlorophyll A

2. Chlorophyll B

3. Carotene

Which of the 3 photopigments found in the group of chlorophyll is the main one?

Chlorophyll A

What colour are Chlorophyll A, Chlorophyll B and Carotene respectively?

Green, blue and orange

What is a photosystem?

Clusters of photopigments and proteins that capture light energy

Where are photosystems found?

In the thylakoid membrane

What is found at the centre of a photosystem?

The reaction centre

What are organisms which photosynthesise classed as?

Autotrophs

What is an advantage of the highly folded structure of thylakoid membranes?

Provides a large surface area for the absorption of photons

Apart from the photopigments from the chlorophyll group can you name one other?

Xanthophyll

Where in a photosystem is chlorophyll a located?

The reaction centre

What is everything except the reaction centre known as in a photosystem?

Antennae complex

What is the role of the antennae complex?

To absorb different wavelengths of light in order to transfer this energy to the reaction centre

What are the two stages of photosynthesis?

Light-dependent and light-independent

Summarise what goes on in the light-dependent stage of photosynthesis?

Energy from sunlight is absorbed and used to form ATP, and hydrogen from water is used to reduce coenzyme NADP to NADPH

Summarise what goes on in the light-independent stage of photosynthesis?

Hydrogen from NADPH and CO2 are used to build organic molecules such as glucose, with ATP supplying the required energy.

Which of the two photosystems absorbs light at lower wavelengths?

Photosystem II

At around what wavelength does PSII absorb light?

680nm

At around what wavelength does PSI absorb light?

700nm

Which of the two photosystems reacts first?

PSII

What are the two possible types of photophosphorylation?

Cyclic and non-cyclic

Briefly explain what happens in cyclic photophosphorylation?

Instead of the electrons from PSI being used to reduce NADP, they can be returned to PSI to form ATP without the input of electrons from PSII

What is not produced during cyclic photophosphorylation?

NADPH

What is photolysis?

Where water molecules are broken down using energy supplied by photons

What is the oxygen-evolving complex?

An enzyme which catalyses photolysis of water

Where is the oxygen-evolving complex found?

PSII

How is ATP produced during the light-dependent reactions of photosynthesis?

High-energy electrons leave the photosystems and travel through electron transport chains, which releases energy that can be used to pump protons so that chemiosmosis can occur

How are the electrons which leave PSII replaced?

Via the electrons produced by the photolysis of water

Do electrons from PSI and PSII pass down the same electron transport chains?

No, different ones

How are the electrons lost from PSI replaced during non-cyclic photophosphorylation?

By electrons released from PSII

How is NADPH produced by PSI?

Electrons leaving the transport chain of PSI are accepted by the coenzyme NADP, along with H+ ions released via photolysis of water, to form NADPH

How does photolysis lead to increased ATP production?

The H+ ions released go through chemiosmosis to power ATP synthase

What are the reactions of the light-independent stage of photosynthesis collectively known as?

The Calvin Cycle

Where does the Calvin Cycle take place?

In the stroma

Briefly, what does the Calvin Cycle do?

Uses NADPH, ATP and CO2 to produce organic molecules such as glucose

Briefly summarise the reactions of the Calvin Cycle

1. RuBP combines with CO2 (via the enzyme RuBisCo) to form an unstable hexose intermediate

2. This splits into two GP molecules

3. Each GP molecule reacts with ATP and NADPH to form a TP molecule

4. One TP molecule is used to synthesis organic molecules, the other is used to regenerate RuBP

What enzyme is used in the Calvin Cycle?

RuBisCo (Ribulose Bisphosphate Carboxylase)

What is a disadvantage of RuBisCo?

It has a high affinity for oxygen, so this can inhibit the Calvin Cycle

What are some organic molecules which can be produced from TP?

Glucose, lipids, amino acids

What are the 3 stages of the Calvin Cycle?

1. Fixation

2. Reduction

3. Regeneration

How many full turns of the Calvin Cycle are required to produce one glucose molecule?

6

How many of the 12 TP molecules produced by 6 turns of the Calvin Cycle are removed to make glucose?

2

What are the three limiting factors of photosynthesis?

1. Light Intensity

2. CO2 availability

3. Temperature

Why is water availability not a limiting factor for photosynthesis?

If water levels were that low then the plant would be in survival mode and not photosynthesising

How is light intensity a limiting factor for photosynthesis?

It is needed as an energy source in the light-dependent stage of photosynthesis so that ATP and NADPH can be produced

How is CO2 availability a limiting factor for photosynthesis?

It affects how much TP can be produced by the Calvin Cycle

How is temperature a limiting factor for photosynthesis?

Higher temperatures increase the rate of reactions catalysed by enzymes such as RuBisCO, but above 25 degrees Celsius photorespiration occurs due to stomatal closure, so the rate of respiration slows

What is photorespiration?

Where oxygen, a competitive inhibitor in RuBisCO, is converted to the toxic compound phosphoglycolate, which must be removed (which is a waste of ATP)

Approximately what % of the products of photosynthesis are lost in photorespiration?

25%